The present invention relates to improvements in an up-shift control apparatus of an automatic transmission, and more particularly to a control of an accumulator back pressure of a friction element to be disengaged in an up-shift transient time.
Japanese Patent Provisional Publication No. 5-248526 discloses a shift change control apparatus of an automatic transmission. When 3-4 up-shift is executed, an improper overlap between a discharging pressure of the low clutch 50 to be disengaged and a charging pressure of the band brake 28 to be engaged invites engine racing or falling of the output shaft torque. Therefore, this conventional apparatus is arranged to provide a low clutch sequence valve 90 for switching the back pressure of a low clutch accumulator 83 from a supply side to a drain side so as to synchronize the charging pressure and the discharging pressure.
However, if the low clutch sequence valve 90 is tuned so as to be switched at a position where the charging pressure and the discharging pressure are overlapped in an interlock condition, the falling of the output shaft torque during the inertia phase starting period becomes large as shown in FIG. 9 and therefore large shock is generated. On the other hand, if the low clutch sequence valve 90 is tuned so as to be switched at a position where the charging pressure and the discharging pressure are slightly overlapped, the engine racing is generated due to the shortage of the engagement capacity during the inertia phase starting period. Therefore, in order to ensure both of preventing the engine racing and reducing the engagement shock, it is necessary to switch the low clutch sequence valve 90 at a timing just before the generation of the engine racing. However, the charging pressure, the discharging pressure and the output shaft torque are fluctuated according to the condition of each shift change operation. Therefore, even if the low clutch sequence valve 90 is tuned so as to be switched at the timing just before the generation of the engine racing as shown in FIG. 10, the engine racing is still generated. Therefore, the low clutch sequence valve 90 is tuned so as to be switched at a position where the switching point is slightly shifted to the interlock condition. However, even by this arrangement, since the hydraulic pressure or output shaft torque is fluctuated, it is difficult to stably and smoothly execute the up-shift without the generation of falling of the output shaft torque or engine racing.